This application claims the priority benefit of Taiwan application serial no. 91136981, filed Dec. 23, 2002.
1. Field of Invention
The present invention relates to a method of manufacturing memory. More particularly, the present invention relates to a method of manufacturing mask read-only-memory (mask ROM).
2. Description of Related Art
Mask read-only-memory (mask ROM) is a type of non-volatile memory that retains data even after power supply is turned off. Due to its versatility, mask ROM has been used for many kinds of computers and electronic products. Conventionally, buried bit lines and words lines are prefabricated in factory as semi-finished products before any order for production is received to save time. When an order is received, a photomask is fabricated according to the specification and then used to carry out the coding process.
A typical coding process includes forming a photoresist layer over a substrate. Thereafter, the photoresist layer is photo-exposed using the photomask fabricated according to the specification. After developing the exposed photoresist, the photoresist layer with coding openings therein is used as a mask to carry out an ion implantation. In this way, ions are implanted into the substrate in regions exposed by the coding openings so that correct codes are set up in the mask ROM.
Following the recent trend of miniaturization, the mask ROM is also required to reach a high level of integration. However, as the size of devices on a chip is shrunk and device density is increased, many problems appear in the conventional method of manufacturing mask ROM.
One of the problems of using a conventional mask ROM manufacturing method is that any alignment error in the process of forming the patterned photoresist layer may lead to a shift in the coding openings. Hence, coding ions may be implanted into a neighboring region instead of the correct coding region leading to the appearance of some coding errors in the finished mask ROM.
Another problem of using the conventional mask ROM manufacturing method is that micro-loading effect due to differences in density of coding openings often leads to size and shape deviation in region with sparse coding openings. If the situation is serious enough, some coding openings may remain close. Since a shift in the critical dimension and shape of coding openings has a direct effect on the location of the code implant regions, precision of the coding process may be severely affected.
To combat the aforementioned problems, pattern on the photomask is often modified with the most advanced processing equipment. However, this will increase processing complexity as well as photomask fabrication cost. Moreover, the turnaround time (TAT) of mask ROM will also be increased due to a longer photomask fabrication turnaround.
Accordingly, one object of the present invention is to provide a method of manufacturing mask read-only-memory (mask ROM) capable of preventing problems caused by a shifting of coding openings from the desired coding region that results from a misalignment of coding mask.
A second object of this invention is to provide a method of manufacturing mask ROM capable of preventing problems caused by a variation in size of coding openings due to a conventional method of fabricating the coding mask.
A third object of this invention is to provide a method of manufacturing mask ROM that produces a higher level of integration but at a lower cost.
A fourth object of this invention is to provide a method of manufacturing mask ROM that has a widen process window.
A fifth object of this invention is to provide a method of manufacturing a mask ROM that has a shorter turnaround time.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a method of manufacturing mask ROM. A buried bit line is formed in a substrate and then a gate and a word line are formed over the substrate. Thereafter, a pre-coding layer with a plurality of pre-coding openings therein is formed over the substrate. The pre-coding openings correspond in position to a plurality of coding regions on the substrate underneath the gate. A filler material is deposited into the pre-coding openings to form a filler layer. A coding mask having a plurality of coding openings is next formed over the substrate. The filler material inside the pre-coding openings that correspond in position to the code openings in the coding mask is removed. The coding mask is removed. Finally, a coding ion implant is carried out using the pre-coding layer and the filler layer as a mask. The ions are implanted into the code region through the pre-coding openings.
According to the embodiment of this invention, a higher precision process is used to form the pre-coding openings in the pre-coding layer while a lower precision process is used to form the coding openings in the coding mask. In addition, the pre-coding layer and the filler layer are made from materials having a different etching rate. The coding layer is fabricated using a material such as silicon oxide, silicon nitride or a metal. The filler layer is fabricated using a material such as spin-coated glass, metal or silicon nitride. If the coding layer or the filler layer is made from a metallic material, the metallic coding layer or filler layer must be removed after the coding ion implant. Furthermore, the filler layer exposed by the coding opening can be removed by carrying out a wet etching process.
Because the pre-coding layer and the filler layer are fabricated using materials having a different etching rate, the coding openings in the mask layer only need to expose a portion of the filler layer above the required coding regions. In a subsequent etching step, the filler layer above the required coding regions can be completely removed by selection. Thus, if the pre-coding openings in the pre-coding layer are precisely aligned to the coding regions in the substrate, the ions in the coding implant process will automatically fall into the desired coding regions. In other words, while patterning the coding openings in the mask layer, even if there is some misalignment in the photolithographic process or some micro-loading effect in the etching process, as long as the coding opening is able to expose a portion of the filler layer above the desired coding region, the etching selectivity of the filler layer and the pre-coding layer can be utilized to form pre-coding openings having uniform shape and size above the pre-coding regions so that implanting ions may self-align with the coding regions.
Since the coding openings in the mask layer is required to expose a portion of the 999 filler layer above the desired coding regions only, processing window is increased so that machinery having a slightly lower precision can be used for the coding process. Moreover, less sophisticated mask-making technique may be employed to form the photomask for patterning the coding openings and to shorten overall turnaround time (TAT). In addition, lower precision photoresist may be used to form the mask layer to lower the production cost even further.
Furthermore, the pre-coding opening in the pre-coding layer has a uniform density. Hence, less sophisticated mask-making technique is required to form the photomask for patterning the pre-coding openings. Moreover, identical pre-coding openings are needed whatever the coding scheme for the memory devices. Therefore, the mask for forming pre-coding openings is applicable to various types of products. Thus, in the fabrication of mask ROM, pre-existing pre-coding mask can be used to pattern the pre-coding layer. In other words, this invention is able to lower overall production cost of a mask ROM, increases processing window and shortens delivery time.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.